Page:Popular Science Monthly Volume 8.djvu/41

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31
HYDROIDS.

that he is still quoted in the latest works as authority. The hydra is found generally in fresh water, though some few species have been discovered, in this country, in that which is somewhat brackish. It loves still or slowly-running water, and attaches itself generally to the under-side of the leaves or to the stalks of aquatic plants. Its body is extremely contractile, and consists, like the oceanic hydroids,

PSM V08 D041 Hydra vulgaris.jpg
Fig. 12.—Hydra vulgaris. (Natural Size.)

in its earliest stage of development, of a simple elongated sac, with an opening which answers the purpose of a mouth. Around the mouth are a series of hollow filaments which it can entirely withdraw, and it then looks like a minute tubercle. The tentacles are roughened by the clusters of thread-cells, or enidæ, already described. The threads have been observed in some instances to be, when extended, as much as eight inches long, and are shot out, it is thought, by the propulsive power of a liquid injected into the central cavity. It grows erect, horizontal, or inverted, as the case may be, and lives only upon animal food. The little creatures are extremely voracious and not over-nice. Trembley observed two hydras attack, at the